Chang earthen dike is located in Kachchh territory of Gujarat province, India, badly affected by the monolithic temblor of strength 6.7 on Richter graduated table on 26th January 2001. Chang Dam, with a design capacity of 6.9 million cmt was constructed in 1963. Chang Dam is located in the bosom of epicentral part. The earthen dike has a entire length of 1227 metre and the maximal tallness of 15.54 metre. The cardinal hearting consists of imperviable clayey fill, augmented with a cardinal masonry nucleus wall. The upstream and down watercourse shells are specified as semi pervious locally available silty sand, flaxen silts and flaxen clays. The liquifiable, all right to medium littorals and silt littorals excavated and replace to a fixed deepness chiefly to cut down escape by under ooze and no efforts was made to take all the potentially, liquifiable alluvial sediment beneath the upstream and downstream embankment subdivisions. During the temblor important liquefaction occurred below the upstream shell zone. A big interlingual rendition slide occurred on the upstream side towards the reservoir ensuing in to dropping the crest as a series of appareled blocks, with maximal crest loss of 6.5 metres. The big incline motions besides produced big clefts and crevices on the upstream face of the dike. Silty sand furuncle ejects from the liquified stuffs were observed at and near to the upstream toe. The major upstream slippage occurred longitudinally along the dike axis with a length of about 85 metres. The elaborate dirt trials of the bing dike subdivision and foundation were carried out. The standard incursion value up to depth of 3.5m is found to be less than 14 so the loose bed is removed and it is replaced by good semi pervious Earth bed. Besides to densify foundation stuff below 3.5mt deepness, wooden hemorrhoids of 15 centimeters diameter in a 1.5 m grid throughout the full deepness of the foundation is driven. Stability analysis of bing dike suggests that it is insecure against temblor burdens. So, extra Earth work is added on u/s and d/s inclines. The present paper describes critically the harm due to temblor and rehabilitation carried out for the dike.
The Chang dike site is situated near small town Kakarva of Bhachau Taluka, 24 km off from Bhachau, Gujarat. Chang is a zoned earthen dike holding an uncoursed rubble masonry nucleus wall in lime howitzer, constructed across river Chang, in 1963. The dike was designed in the 50aa‚¬a„?s, without accounting for the seismal considerations. The overall length of the dike is 1266m. Maximal tallness of the dike is 15.54m, above deepest riverbed degree. The dike caters for a catchments country of 45.30 sq kilometer. The storage capacity of the reservoir is 6.943Mm3. The dike was severely damaged due to temblor. From the extent of the harm it falls in class III, which require rehabilitation and strengthening.
The strategy is supplying H2O for irrigation to an country of 486 Ha. The mean rainfall of the country is 36.50 centimeter, the upper limit and minimal rainfall being 70 centimeter and 5 centimeter severally.
2.0 Damagess Caused Due to Earthquake
Figure 1 shows a design cross-section for the maximal tallness, dam subdivision across the watercourse channel. The cardinal hearting or nucleus zone consists of imperviable ( clayey ) fill, augmented with a cardinal masonry nucleus wall. The environing upstream and downstream shells are specified as semi pervious earthfill, and this reportedly consist of locally available silty sand, flaxen silts, and flaxen clays. The dike was constructed prior to local consciousness of the possible jeopardies associated with dirt liquefaction ; it was non designed/constructed to take possible liquefaction into history. Excavation and replacing of flaxen watercourse channel alluvial sediment up to a fixed deepness was carried out, chiefly to cut down escape by under ooze, and no effort was made to take all the potentially liquifiable alluvial sediment from the foundation during the building of dike in the twelvemonth 1963. As a consequence, potentially liquifiable, all right to medium littorals and silt littorals were left in topographic point beneath the upstream and downstream embankment subdivisions.
During the temblor, important liquefaction occurred beneath the upstream shell zone, where the low dead H2O reservoir pool saturated the alluvial sediment and the base of the shell. A big interlingual rendition slide occurred on the upstream side towards the reservoir. This pulled apart the cardinal part of the embankment, dropping the crest as a series of graben like blocks, with maximal crest loss of up to 6.5 metres. The big incline motions besides produced big clefts and crevices on the upstream face of the dike. Silty sand furuncle expulsions were observed at and near to the upstream toe.
Extra liquefaction, once more as evidenced by all right sand and silty furuncles ejecta, was observed next to a little agricultural pool near the downstream toe, and minor bulging or sidelong interlingual rendition was observed at the downstream toe. The major upstream slippage occurred longitudinally, with a length of about 85 metres along dam axis. In add-on, comparatively minor longitudinal snap occurred along the crest of the lesser height embankment subdivisions off from the deep channel subdivision.
Figure 3 shows a position of the failed upper limit height channel subdivision of the embankment. Fig. 4 shows cross clefts on the crest. This graben block, originally portion of the crest, has displaced vertically about 6m, in add-on to take downing the evident crest by more than 6m, clefts and crevices reduced the effectual crest height even further, and the imperviable nucleus zone and the upstream slide motion massively disrupted masonry nucleus wall. It is assumed that the transverse clefts in the gorge part of the dike are due to the presence of masonry nucleus wall. The record of earlier experience indicates that the embankment and the masonry nucleus wall did non vibrate together when the dike foundation was shaken, therefore doing break and snap in the part.
3.0 Immediate Measures
The dike was opened to let safe transition of floodwaters, as it was non possible to convey it to safe phase within the really short period available before monsoon of 2001. A cut of 188m was made in the gorge part for this intent, which was considered safe for the transition of the inundation.
Rehabilitation & A ; Reconstruction of Dam
The rehabilitation/reconstruction works of the dike included:
I ) Field Probes
two ) Design facets
three ) Construction.
Detailed dirt probes were conducted and soil trials of bing dam subdivision, borrow country dirts for Restoration and foundation dirts were carried out at Gujarat Engineering Research Institution ( GERI ) . Based on trial consequences the design parametric quantity of different zones of the earthen dike subdivision was worked out.
The seismal coefficient of 0.16g for the design with importance factor 2.5 has been considered. The revised subdivisions arrived at after taking into considerations of seismal co-efficient are much wider and monolithic. The stableness analysis showed that factors of safety under assorted conditions of lading are within required bounds.
B aa‚¬ ” Treatment against liquefaction:
In the cut part ( Gorge Portion ) , loose dirt was removed up to 3.5 m deepness. To densify the foundation stuff, cock driven level underside wooden hemorrhoids of 15 centimeters diameter up to sway in cut part and in a breadth of dam place plus 10 m on either side were driven. The figure of hemorrhoids driven was 4438, with their entire length 23935 Rmt. The deepness of hemorrhoids varied with regard to the stone strata and the lower limit and maximal deepness boulder clay refusal of drive had been 2.20 to 6.70 m.
In other part of the Earth dike, on u/s side near the toe bing loose dirt was removed in 12.5 m broad and 1.0 m deepness and replaced by pervious stuff. Above this compacted bed, lading berm of 11.0 m and on u/s side stone toe was provided as a step against liquefaction.
The building started with the jungle clearance and procurance of stuffs required for different zones of the dike.
Compaction of Earthen Dam Embankment
The quantum of earthwork involved has been:
( I ) Clay core stuff ( IP ) / Hearting = 51,155 semen
( two ) Casing ( SP ) stuff = 1, 00,502 semen
( three ) Sand filter = 7,744 semen.
I ) Clay nucleus / Hearting Material
The imperviable stuffs for cardinal nucleus was obtained from comfortable borrow countries. These powdered dirts largely comprise of clay silts in changing per centums. The conventional physical word pictures like step, atterberg bounds, shear parametric quantities, coefficient of permeableness, monitor denseness every bit good as squeezability indicated their acceptableness as nucleus stuffs. The clay content largely varied from 18-25 % measure uping as CL / CI stuff. Proper compression was done to accomplish an temblor immune dike by deploying appropriate compacting equipment. Normally loose bed of 20 to 25 centimeter was compacted to around 15 centimeter by accomplishing dry denseness of 95 % of MDD. Verification of compression was done for each bed.
two ) Casing Material
Similarly the casing stuff had been investigated from nearby borrow countries for its suitableness in casing zone. This stuff contained 5-10 % clay measure uping as SC / SM stuff. For the casing zone, usually loose bed of 30 centimeter was compacted to around 25 centimeter. The wet content was kept 8-10 % and 6-8 base on ballss of vibratory roller of 11-22 metric tons, were found plenty for the necessary compression. Confirmation of compression was done for each bed.
Fliping on the Upstream Slopes of Dam Embankment
The manus placed riprap was provided on the u/s incline of the dam embankment as indicated in the drawing. The riprap stuff consisted of the most lasting stone fragment of sanctioned quality selected from local preies. Care was taken to guarantee that at least 80 % of the country of riprap had rocks weighing more than 25 kilogram.
Area of u/s pitching is about 52110 sqm. Headings in M10 concrete of size 150x150x600 millimeter have been provided normal to incline stick outing at least 15 centimeters above general surface of the riprap to disperse beckon energy of reservoir H2O.
Fliping on Downstream Slope
Kachchh part is holding waterless clime and heavy cyclonic rains in monsoon ; as such pitching is provided on d/s inclines to forestall rain cuts. The size and qualities of the rocks is the same as that provided for u/s incline. The quantum of work for d/s pitching has been of the order of 13000 Sqm.
Graded filter Under Riprap Work
Graded filter has been placed underneath the riprap on the u/s inclines of embankment and in the u/s incline of the stone toe dividing it from the Earth fill. The filter under upstream riprap has been placed in two different beds i.e. the finest over the bomber class followed by coarser stuffs conforming to the specifications.
In the Chang dike which is shaken by terrible temblors ; two primary types of harm have occurred: longitudinal clefts at top of the embankment, and crest colony. The crest colony was non normally great plenty to endanger failure by dominating, as the reservoir H2O degree was rather low at the clip of temblor.
Chang dike is founded upon potentially liquifiable alluvial sediment in the watercourse channel that it serve to the dike. During the temblor, liquefaction beneath the earthen dike subdivision led to sidelong spreading and slow failure. These low-density stuffs are one of the grounds of complete or partial failure Chang dike.
The harm to the dike appears to hold been caused chiefly by the horizontal constituent of the temblor motion in the upstream-downstream way ; that is, the way transverses to the longitudinal axis of the dike. Probably the amplitude and the acceleration of the horizontal constituent of the motion of the crest in this way is much larger than the motion of the foundation, and this resulted in a snappy action of the dilutant top of the dike, doing longitudinal clefts in dike.
Chang dike is a zoned Earth dikes with cardinal concrete or masonry nucleus wall. The amendss of Chang Dam indicate that the embankment and the concrete wall did non vibrate together when the dike foundation was shaken.
The revised subdivision arrived after taking in to consideration seismal coefficient as 0.16 g and importance factor as 2.5 was much wider and monolithic.
The intervention against liquefaction was carried out in two ways. First, in the cut part loose dirt was removed up to 3.5 meitneriums deepness and foundation is densified by driving 4438 wooden hemorrhoids.
After the rehabilitation work the dike has been efficaciously utilised today for the intents for which it was constructed.